The move comes after Transperth trialled a diesel-electric hybrid bus and found the technology was not yet ready for wider use on the local network.

Transport Minister Dean Nalder said the 18-month trial assessed the hybrid's on-road handling, maintenance requirements and the economic and social costs of the technology.

"A lack of quickly available parts, the need to re-train staff for the new technology and the fact that the cleanest diesel bus in the world is already available and tested in this market was overwhelming evidence that, for the time being at least, diesel buses remain the best choice for the Transperth network," Mr Nalder said.

The Minister said the cost over time of hybrid buses did not represent value for money for the taxpayer and was much higher than the Euro6 clean diesel bus options.

Mr Nalder said the Government had a strong track record of investing in cutting-edge technology, including the decision to undertake this diesel-electric hybrid bus trial in order to assess the status of this technology.

"We will continue to keep our eye on what the market can offer in terms of low-emission buses, but for the time being we will stick with ultra clean diesel. Euro5 is the current Australian standard, so we are well-ahead of the curve by ordering Euro6 buses for the CAT fleet."

The State Government provides free CAT buses on four routes, using about 40 buses. There are about 29,000 boardings on the 600 bus trips on a typical weekday. The operating costs of the service are funded through the Perth parking fund.

Fact File

$18.2m investment in Euro6 buses to replace 32-strong CAT fleet

In addition to $58.5m for 70 articulated Euro6 buses for the wider Transperth network

Euro6 buses are cleaner than current Australian standard, Euro5

They are assembled by staff at Volgren in Malaga in Perth's north-eastern suburbs